1. Field of the Invention
The present invention relates to a hydraulic brake apparatus for a vehicle having both a hydraulic booster function which can assist a brake operation of a driver by a hydraulic pressure from a high pressure hydraulic source and a braking force control function by an electronic control.
2. Description of Related Art
In a hydraulic brake apparatus for a vehicle having both the hydraulic booster function and the braking force control function such as Anti-lock Brake System (ABS) or an Electronic Stability Control (ESC) by an electronic control, a hydraulic pressure generated by a power driven pump is feedback to an output path from a master cylinder while the braking force control by the electronic control is performed. Since the output path is closed during operation of braking, a hydraulic pressure generated by the master cylinder (hereinafter referred to as “MC pressure”) is increased to be higher than necessary pressure due to liquid amount by pump-back. This situation is not desirable from the viewpoint of the safety, and also, durability of the components in a hydraulic circuit is deteriorated.
In consideration of the above situation, JP H08-133039A describes a technique that an excessive pressure is relieved by providing a relief valve in a discharge path from a pump (see, paragraph 1, line 19).
Furthermore, JP 2001-260860A describes a technique that when a pressure in a main path communicating with a discharge port of the pump is abnormally high, an electronic gate valve provided in the main path is controlled to relieve the excessive pressure.
The relief valve described in JP H08-133039A is configured to protect components or hydraulic paths from abnormal pressure and an actuation pressure thereof is set slightly high so as not to affect a desired brake performance. For this reason, in the brake apparatus equipped with the relief valve, an excessive MC pressure could be generated to deteriorate the durability of hydraulic components in the hydraulic path. In addition, although the variation and increase of the MC pressure is thought to disturb the control using a sensor for detecting the MC pressure, there is no countermeasure to solve such a problem.
In the technique described in JP 2001-260860A, the relief valve is not employed, and thus, the above problem is not occurred. However, this technique requires an electronic gate valve and a drive circuit for the electronic gate valve. The electronic gate valve is required to be subject to a pulse modulation control, so that the electric power is always supplied to the electronic gate valve while the braking force control is performed. Therefore, there is a problem that power consumption is increased and the size of the brake apparatus becomes larger for mechanism of preventing over heat.